1. Field of the Invention
The present invention relates to an image processing method for a Scanning Electron Microscopy (SEM) image and to a critical dimension measuring method for a pattern obtained by detecting secondary electrons and reflected electrons from the pattern, as well as to a critical dimension measuring equipment, and, in particular, to a critical dimension measuring method and equipment, wherein, when measuring the critical dimensions of a finely detailed pattern or the like in a VLSI manufacturing process, if the measured pattern is either circular or elliptical the diameter, major axis, and minor axis are measured automatically with high precision and a judgment as to whether the measured pattern is circular or elliptical is made automatically, so that the precision of the measurement and the operation of measuring the critical dimensions are improved.
2. Description of the Prior Art
Conventionally, a pattern recognition method and a pattern measurement method using an electronic beam are utilized for measuring the critical dimensions of finely detailed patterns during a process for manufacturing Very Large Scale Integrations (VLSIs) and the like. At this time, the electron optics parameters are set at a low acceleration voltage (1 kV or less) and a low beam current (10 pA or less) with the object of reducing irradiation damage to the sample and preventing charge-up.
In addition, a frame integration processing or a spatial filtering processing or the like is performed to improve the S/N ratio of the resulting SEM image.
Furthermore, various algorithms for dimension measurement (for example, a threshold value method, linear regression method, maximum differential method, or the like) are used, and finely detailed critical dimension measurement is carried out based on the SEM image obtained in this manner.
In a VLSI manufacturing process, if the pattern is line-shaped, it is possible to obtain a measurement repeatability of 0.01 .mu.m or less with such conventional measurement methods. However, under present conditions, not only are the design rules for the design data very detailed, but, even if the pattern is square or rectangular, it is treated as a circular or an elliptical shape in a lithography process.
When the critical dimensions for a circle or an ellipse are measured in this manner, the diameter is measured in the case of a circle, and the major axis or minor axis dimension is measured in the case of an ellipse.
Conventionally, when critical dimensions are measured for such circles or ellipses, the following problem areas are encountered.
First, in the case of a circle, in judging the diameter, profile data for any line part in the diametrical direction of the circle is extracted and the dimensions are measured, for each line. The maximum value is normally taken as the diameter. The repeatability of the measurement is good because it is possible to average the profile data for any part of the line for a line shape.
However, with a circle, the profile data for the lines cannot be adequately averaged because it is necessary to measure the dimensions of each line. Therefore the deviation in the dimensional values cannot be disregarded and in practice there is a problem as to whether the average value corresponds to the diameter.
In the case of an ellipse, the major axis direction or the minor axis direction must agree with the beam scanning direction. If the pattern is sufficiently large it is possible that these directions can be made to agree, but with a finely detailed critical there are many cases in which such agreement is extremely difficult. Specifically, the operability is poor. Accordingly, there is a high possibility of erroneous measurement so that the major axis dimension is too short or the minor axis dimension is too long. In the same manner, in the case of a circle it is not possible to average the data for each line, therefore the deviation in the measurements cannot be disregarded.
As outlined above, with a conventional critical dimension measurement equipment and method:
(1) When the measured pattern is circular, the dimensions of every line must be measured, and when compared with a straight line pattern, the deviation of the dimension values cannot be disregarded because the data for each line cannot be adequately averaged. PA1 (2) When the measured pattern is elliptical, the major axis direction or the minor axis direction must conform to the beam scanning direction. With a finely detailed pattern there are many cases in which it is extremely difficult to obtain such agreement. There is therefore a high possibility of erroneous measurement whereby the major axis dimension is too short or the minor axis dimension is too long. In the same manner as in the case of a circle as outlined above, it is not possible to average the data for each line, therefore the deviation in the measurements cannot be disregarded. PA1 irradiating means for irradiating electron beam onto a measured pattern; PA1 detecting means for detecting secondary and reflected electrons from the measured pattern; PA1 filtering means for inputting image data of the secondary and reflected electrons obtained from the detecting means and implementing spatial filtering processing of this image data and storing the results in a first memory; PA1 histogram processing means for inputting image data after spatial filtering processing stored in the first memory, implementing histogram processing of this image data, and storing the results in a second memory; PA1 threshold value detection means for inputting the results of the histogram processing stored in the second memory, obtaining a threshold value by automatically separating classes in the histogram obtained by the histogram processing based on a discriminant criteria method, and storing the results in a third memory; PA1 three-value conversion processing means for inputting the threshold value stored in the third memory, implementing three-value conversion of the image data stored in the first memory obtained after spatial filtering processing based on the threshold value, and storing the results in a fourth memory; PA1 first calculation means for inputting image data stored in the fourth memory obtained after three-value conversion processing, obtaining the area and perimeter of the bottom section of the pattern based on this data, and storing the results in a fifth memory; PA1 second calculation means for inputting the area and the perimeter of the bottom section of the pattern stored in the fifth memory, obtaining the diameters of the patterns based on this data, and storing the results in a sixth memory; and PA1 pattern shape recognition means for automatically deciding whether the pattern is circular or elliptical based on the pattern diameter stored in the sixth memory, calculating the diameter of the circle based on the area if the pattern is circular, and calculating the major axis and the minor axis of the ellipse based on the area and the perimeter if the pattern is elliptical. PA1 a first process for spatial filtering processing of Scanning Electron Microscope (SEM) image data obtained by scanning electron beam on a pattern to be measured; PA1 a second process for histogram processing of image data after spatial filtering processing; PA1 a third process for obtaining a threshold value by automatically separating classes obtained by the histogram processing based on a discriminant criteria method; PA1 a fourth process for three-value conversion of the image data after spatial filtering processing based on the threshold value; PA1 a fifth process for obtaining the area and perimeter of the bottom section of the pattern based on the image data after the three-value conversion process; PA1 a sixth process for obtaining the diameters of the patterns based on the area and perimeter of the bottom section of the pattern and PA1 a seventh process for automatically deciding whether the pattern is circular or elliptical based on the pattern diameter, calculating the diameter of the circle based on the area if the pattern is circular, and calculating the major axis and the minor axis of the ellipse based on the area and the perimeter if the pattern is elliptical.